Canberra WK129 had undertaken a secret radar test which involved a flight from RAF Pershore in Worcestershire, north west over the Carneddau mountains in Snowdonia to Anglesey and back.
The bomber flew over the radar hut which was on the summit of Drum and reported back to base while in the vicinity of Puffin Island off the eastern tip of Anglesey. The bomber then turned and headed back to base, but just 10 miles or so into the return leg the Canberra crashed near the summit of Carnedd Llewelyn.
Debris in the stream
Bits of wing
(Source; No Landing Place Vol 2, Edward Doylerush, 1999)